The 1999 Buick Riviera is available as a 2-door, 5-seat coupe. Starting at $33,820, the 1999 Buick Riviera Base is powered by a 3.8L, 6-cylinder engine mated to an automatic transmission that returns an EPA estimated 18-mpg in the city and 27-mpg on the highway. The top of the line 1999 Buick Riviera GM BuyPower, priced at $33,820, is powered by a 3.8L, 6-cylinder engine mated to an automatic transmission that returns an EPA estimated 18-mpg in the city and 27-mpg on the highway.